He has been hospitalized in Pretoria since June 8 for a recurring lung infection. Previously, authorities had described his condition as serious but stable.

"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable. He is in good hands," President Jacob Zuma said in a written statement, referring to Mandela's tribal name.

Mandela, 94, has become increasingly frail over the years and has not appeared in public since South Africa hosted the World Cup in 2010.

The anti-apartheid hero has been in and out of the hospital in recent years.

His history of lung problems dates to when he was a political prisoner on Robben Island during the apartheid era, and he has battled respiratory infections.

Considered the founding father of South Africa's democracy, Mandela became an international figure while enduring 27 years in prison for fighting against apartheid, the country's system of racial segregation.

"The African National Congress joins the presidency in calling upon all of us to keep President Mandela, his family and his medical team in our thoughts and prayers during this trying time," it said.

In 1993, Mandela and then-South African President F.W. de Klerk jointly won the Nobel Peace Prize.

The iconic leader was elected the nation's first black president a year later, serving only one term, as he had promised.

Even as he has faded from the spotlight, he remains popular and is considered a hero of democracy in the nation. Last year, South Africa launched a new batch of banknotes with a picture of a smiling Mandela on the front.

Mandela's impact extends far beyond South African borders. After he left office, he mediated conflicts in Africa and the Middle East.

Word of his worsening health drew global expressions of concern.

"We have seen the latest reports from the South African government that former President Mandela is in critical condition," Caitlin Hayden, a U.S. National Security Council spokeswoman, said in a statement. "Our thoughts and prayers are with him, his family and the people of South Africa."